Chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been applied as techniques for depositing thin films for semiconductor devices because they enable the achievement of conformal films (metal, oxide, nitride . . . etc) through fine tuning of parameters during the process. The film growth is mainly controlled by the chemical reaction of metal-organic compounds (precursors), so the development of optimum precursors is essential under prediction of its property and reaction process. Precursors have been developed to attain required properties based on its specific application to certain types of film.
Several intrinsic properties of precursors should be considered before using them as molecules for CVD and ALD processes. First, liquid form and/or sufficient vapor pressure are necessary for easy delivery of the precursor in a gas phase into the reaction chamber from the containing vessel. Second, long term thermal stability in storage conditions and at delivery conditions is required. Thermal stability in the gas phase is also required to avoid incorporation of impurities into the film. Third, strong reactivity toward reaction gases, such as ammonia or oxygen, is required for the precursor to be readily converted into the desired film on the sample substrate. Another important requirement of precursor to be considered at the step of precursor design is to control impurities in the film, which usually originate from the ligand during the deposition process.
Silicide process is considered essential in the complementary metal-oxide-semiconductor (CMOS) for lowering contact resistance as the devices are scaled down. TiSi2 and CoSi2 have been extensively investigated as contact material. However these materials have been reported to have serious problems at sub-65 nm technology node. In addition sheet resistance of TiSi2 steeply increases with scaling down, which routinely is called narrow line effect. Although CoSi2 has immunity to narrow line effect, the greater consumption of Si is a major concern in forming silicides with the decreased junction depth.
Meanwhile, NiSi has become a serious candidate as a contact material thanks to its immunity to narrow line effect, low silicon consumption, and low resistivity. NiSi can be formed through a self-aligned silicide process. However physical vapor deposition (PVD) is becoming unsuitable for nanoscale deposition due to poor step coverage in high aspect ratio contact holes. Therefore Atomic Layer Deposition (ALD) is a promising deposition method for this purpose due its excellent conformality in 3D structures and atomic thickness controllability. Until now, although several studies on nickel ALD have been performed, they have shown limitations such as complex process, high impurity content, or low growth rate.
Alternatively Nickel oxide (NiO) has received lots of attention in the semiconductor industry. Its resistance switching characteristics of NiO thin films show its potential applications for the next generation nonvolatile resistive random access memory (ReRAM) devices.
Heindirk torn Dieck reported two DAD ligands in their dianionic form (DAD(II)) coordinate with titanium in its +IV oxidation state, as shown in the formula Ti(IV)(iPr2-DAD(II))2 and Ti(IV)(tBu2-DAD(II))2 (Heindirk tom Dieck at al., Inorganica Chimica Acta, 177, 1990, Pages 191-197). CVD using Ti(IV)(tBu2-DAD(II))2 was also reported (S. Van Der Ven et al., J. De Physique IV, Colloque C3, supplement au Journal de Physique 11, 3, 1993).
The synthesis of homoleptic nickel (0) molecules such as Ni(0)(R2-DAD)2, where the nickel has the 0 oxidation state have already been reported (H. Tom Dieck Z. Naturforsch. 36b, 814-822, 1981). Two different methods were used for the preparation of these molecules. The first method is based on a ligand exchange by reacting the nickel (0) starting material Ni(COD)2 with two equivalents of the corresponding neutral diazabutadiene. The second method consists of reducing the nickel (+II) starting material NiBr2(DME) with sodium in presence of the corresponding neutral diazabutadiene.
The synthesis of homoleptic nickel (+II) molecules such as Ni(+II)(R2-DAD)2, where the nickel has the (+II) oxidation state have also been reported (T. J. Knisley Organometallics, 2011). In this case, the nickel (+II) starting material Ni(+II)Cl2.CH3CN was reacted with two equivalents of lithium diazadienyl which was beforehand prepared from the corresponding diazabutadiene reacted with one equivalent of freshly cut lithium metal.
Winter et al. report ALD deposition of NixN on 500 nm thick thermal SiO2 using Ni(tBu-DAD)2 and anhydrous 1,1-dimethylhydrazine (WO2012/027357). Self-limited film growth was achieved with pulse lengths greater than or equal to 4.0 seconds as evidenced by a constant growth rate of 0.7 A/cycle, but only at temperatures between 225° C. and 240° C.
Han reports CVD deposition of Ni films on silicon wafers using Ni(iPr-DAD)2 (WO2012/067439). Auger spectroscopy analysis revealed that the resulting film contained approximately 10% carbon.
The deposition of nickel containing films (pure nickel, nickel oxide or nickel silicide) in CVD or ALD mode remains a challenge (high C, N or O content) due to the poor availability of suitable precursors. Applicants believe that no oxygen-free precursors allow deposition of pure nickel films in CVD or ALD (thermal or plasma mode) at temperatures lower than 250° C. or 150° C. using an oxygen-free reaction gas. The nickel precursors currently available are too stable and therefore do not allow deposition at low temperature
A need remains for nickel containing precursors suitable for CVD or ALD in an oxygen-free process. Desirable properties of the nickel containing precursors for these applications are: i) liquid form or low melting point solid; ii) high volatility; iii) sufficient thermal stability to avoid decomposition during handling and delivery; and iv) appropriate reactivity during CVD/ALD process. At the same time, in order to allow the deposition at low temperature the thermal stability should not be too high. 0 oxidation state metal precursors are known to have less thermal stability compared to their oxidized forms.